294 
MR. R. T. GLAZEBROOK ON PLANE WAVES IN A BIAXAL CRYSTAL. 
The direct reading was taken several times during each day’s work to avoid error. 
The method of observing was as follows :—I set the telescope to a given scale 
reading, and turned the prism till the image of the slit seen by reflexion at one of its 
faces coincided with the needle point. 
The difference between the direct reading and the known one to which the telescope 
was set gave the supplement of twice the angle of incidence, whence the angle of 
incidence is easily obtained. I then turned the telescope till one image of the slit seen 
by refraction through the prism coincided with the needle point and took the reading. 
The difference between this and the direct reading gave the deviation for that wave. 
The observation was then repeated for the second wave. 
I then reset the telescope, so that its axis made an angle of 4° with its former 
position, and took a similar set of observations. This gave a series of deviations 
corresponding to angles of incidence increasing by 2°. 
Just in the neighbourhood of the optic axes, observations were made at every 1° of 
incidence. 
For the first prism, each observation in the set finally chosen was repeated three 
times, and the mean of the results taken. 
The difference between this mean and any observation was only in a few cases as 
great as 10b 
In the case of the second prism, the observed values of the deviation on different 
days for the same angle of incidence agreed so closely, rarely differing by more than 
20", that it seemed unnecessary to take a third set. 
I found also that the refracted images were much more distinct for angles of 
incidence greater than that for minimum deviation, than for angles less. 
On passing through the position of minimum deviation, I therefore reversed the 
prism, so that the face of incidence became that of emergence. 
Pig. 1. 
Thus, if A B C D be the ray undergoing minimum deviation, for a ray such as 
P B Q R, I made P B the incident ray, Q R the emergent ; while for a ray like 
S B T U, I reversed the prism, so that U T became the incident, B S the emergent ray. 
The cause of the increased distinctness lies in the fact that as the anode of incidence 
O 
increases from that giving minimum deviation, the breadth of the image of the slit 
decreases, and vice versd. 
